<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
</head>

<body>
    <input type="file" id="upload">
    <script>
        const ACCEPT = ['image/jpg', 'image/png', 'image/jpeg'];
        const MAXSIZE = 3 * 1024 * 1024;
        const MAXSIZE_STR = '3MB';

        function convertImageToBase64(file, callback) {
            let reader = new FileReader();
            reader.addEventListener('load', function (e) {
                const base64Image = e.target.result;
                callback && callback(base64Image);
                reader = null;
            });
            reader.readAsDataURL(file);
        }

        function compress(base64Image, callback) {
            let maxW = 1024;
            let maxH = 1024;
            const image = new Image();
            image.addEventListener('load', function (e) {
                let ratio; // 图片的压缩比
                let needCompress = false; // 是否需要压缩
                if (maxW < image.naturalWidth) {
                    needCompress = true;
                    ratio = image.naturalWidth / maxW;
                    maxH = image.naturalHeight / ratio;
                } // 经过处理后，实际图片的尺寸为 1024 * 640
                if (maxH < image.naturalHeight) {
                    needCompress = true;
                    ratio = image.naturalHeight / maxH;
                    maxW = image.naturalWidth / ratio;
                }
                if (!needCompress) {
                    maxW = image.naturalWidth;
                    maxH = image.naturalHeight;
                } // 如果不需要压缩，需要获取图片的实际尺寸
                const canvas = document.createElement('canvas');
                canvas.setAttribute('id', '__compress__');
                canvas.width = maxW;
                canvas.height = maxH;
                canvas.style.visibility = 'hidden';
                document.body.appendChild(canvas);

                const ctx = canvas.getContext('2d');
                ctx.clearRect(0, 0, maxW, maxH);
                ctx.drawImage(image, 0, 0, maxW, maxH);
                const compressImage = canvas.toDataURL('image/jpeg', 0.5);
                callback && callback(compressImage);
                canvas.remove();
            });
            image.src = base64Image;
        }

        function uploadToServer(compressImage) {
            console.log('upload to server...', compressImage);
            const _image = new Image()
            _image.src = compressImage
            document.body.appendChild(_image)
        }
        const upload = document.getElementById('upload');
        upload.addEventListener('change', function (e) {
            const [file] = e.target.files;
            if (!file) {
                return;
            }
            const {
                type: fileType,
                size: fileSize
            } = file;
            if (!ACCEPT.includes(fileType)) {
                alert(`不支持[${fileType}]文件类型！`);
                upload.value = '';
                return;
            } // 图片类型检查
            if (fileSize > MAXSIZE) {
                alert(`文件超出${MAXSIZE_STR}！`);
                upload.value = '';
                return;
            } // 图片容量检查
            // 压缩图片
            convertImageToBase64(file, (base64Image) => compress(base64Image, uploadToServer));
        })
    </script>
</body>

</html>